\begin{figure}[H]
\centering
\begin{tikzpicture} %[scale=.6,transform shape] %<-- uncomment this to see the effect
\tikzstyle{level 1}=[sibling distance=52mm] 
\tikzstyle{level 2}=[sibling distance=18mm] 
\tikzstyle{level 3}=[sibling distance=18mm] 

 \node{$(\mbox{EX}~(\mbox{OR}~A~B))$}
  child{node{$(\mbox{OR}~A~B)$} child{ node{$A$}} child{node{$B$}}};
 \end{tikzpicture}
\caption{Arbre de la formule $EX(\$A~\|~\$B)$}
\label{fig:ValidationAnalyse}
\end{figure}